Multipod Nanostructures and Methods

ABSTRACT

Methods of forming metal multipod nanostructures. The methods may include providing a mixture that includes a metal acetylacetonate, a reducing agent, and a carboxylic acid. The mixture may be contacted with microwaves to form the metal multipod nanostructures. The methods may offer control over the structure and/or morphology of the metal multipod nano structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/646,055, filed Mar. 21, 2018, which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under CHE 1608364awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND

According to the relevant literature, the growth of metal nanoparticlestypically follows an autocatalytic 2-step Finke-Watsky mechanism,wherein the first step is a relatively slow reduction step (k₁) of acationic precursor in solution by a weak reducing agent, followed by asecond, relatively fast reduction step (k₂) of the cation precursor at asurface of the growing nanoparticle.

In the autocatalytic mechanism, usually k₂>>k₁, which typically causesgrowth to be dominated by the nanoparticle surface. Factors that canlead to hyper-branching may include the nuclei morphology, the facetstability for a given metal, the reaction conditions, and the faceselective binding of ligands. While spherical particles are desirablefor certain applications, branched metal nanoparticles having a highsurface area may be attractive for a number of applications, includingcatalysis and plasmonic applications.

Several mechanisms have been suggested to explain multipod growth. Oneof the suggested mechanisms is driven by preferential growth of the(111) facet, which is initiated by an overgrowth mechanism occurring onnucleated cubic (fcc) or pyramidyl (hcp) nanometal seeds.

Recent studies have revealed that branching, at least for face-centeredcubic (fcc) metals, may be achieved by initial overgrowth on high energyvertex and edge sites corresponding to the (111) facets. It has beenrevealed that morphology control may be achieved by facilitatingpreferential growth along the (111) facet, wherein such growth isinitiated by (111) overgrowth on nucleated cubic (fcc) nanometal seeds.Recent studies have applied this concept to a number of metals, and thestudies have shown that materials with a rapid metal reorganization canlead primarily to isotropic structures (e.g., Au, Ag, Cu), whilestructures with a slow reorganization can grow readily as anisotropicstructures (e.g., Ni, Pd, Pt).

Isolating only multipods can be a complex process, and typical reactionsyield a distribution of morphologies. Typically, convective reactionsrequire careful control of the temperature, the ligand content, and/oroften require long reaction times in a reducing environment (H₂).

The use of microwave (MW) chemistry for nanomaterials synthesis hasattracted attention, due, at least in part, to the enhancement ofreaction rates and/or reproducibility of the materials when carried outin a single mode MW reactor. The observed enhancement is likely not dueto the energy absorbed per MW photon, as it does not contain enoughenergy to break a bond, but rather to enhanced growth rates, whichreflect the evolving dielectric loss tangent for a growing nanoparticle.The size-dependent loss tangent may reflect the repolarization of theelectric field, as disclosed within the Maxwell-Wagner (M-W) model. Suchpolarization effects are believed to be enhanced at sharp tips andedges, leading to enhanced heating at these sites in a growing metal.This is commonly referred to as a “lightning rod” effect because theshart tips and edges can generate very high electric fields in theirvicinity due to the surface charge density in those regions.

Although multipod Ni-, Pt-, and Pd nanoparticles have been reported, thecurrent methods of producing these multipods generally do not permitsystematic control of arm length, aspect ratio, or a combinationthereof. Moreover, the methods often require long reaction times and/orone or more other features that can limit their scalability.

In order to realize the benefits of nanostructured materials, includinghyper-branched nanostructured materials, there remains a need forsimple, economical, scalable, reliable and/or efficient routes for theirsynthesis, including routes that rely on microwave-driven chemistry,which may provide control over nanomaterial growth and/or morphology.

BRIEF SUMMARY

Provided herein are microwave-driven methods of forming metal multipodnanostructures that provide control over nanomaterial growth,morphology, or a combination thereof. Also provided are metal multipodnanostructures.

In one aspect, methods of forming metal multipod nanostructures areprovided. In some embodiments, the methods include [i] providing amixture disposed in a microwave reaction vessel, wherein the mixtureincludes a metal acetylacetonate, a reducing agent that includes aC₁-C₃₀ hydrocarbyl-amine, and a carboxylic acid of formula (A)

wherein R is a monovalent C₁-C₃₀ hydrocarbyl; and [ii] contacting themixture with microwaves to form the metal multipod nanostructures. Insome embodiments, the contacting of the mixture with microwaves includes(a) contacting the mixture with a first plurality of microwaveseffective to heat the mixture to a first temperature, (b) reducing thefirst temperature of the mixture to a second temperature, and (c)contacting the mixture with a second plurality of microwaves for a timeeffective to heat the mixture to the first temperature. Steps (b) and(c) may be repeated from 1 to 8 times, or more.

In some embodiments, the methods include (i) providing a mixturedisposed in a microwave reaction vessel, the mixture including (a)nickel acetylacetonate, (b) oleylamine, and (c) oleic acid, (ii)contacting the mixture with a first plurality of microwaves effective toheat the mixture to a first temperature, wherein the first temperatureis about 260° C. to about 300° C., (iii) reducing the first temperatureof the mixture to a second temperature, wherein the second temperatureis about 220° C. to about 250° C., (iv) contacting the mixture with asecond plurality of microwaves for a time effective to heat the mixtureto the first temperature, (v) reducing the first temperature of themixture to the second temperature, and (vi) contacting the mixture withthe second plurality of microwaves for the time effective to heat themixture to the first temperature. The time effective to heat the mixtureto the first temperature with the second plurality of microwaves may beabout 1 minute to about 20 minutes. In some embodiments, the methodsalso include repeating steps (v) and (vi) from 1 to 20 times.

Additional aspects will be set forth in part in the description whichfollows, and in part will be obvious from the description, or may belearned by practice of the aspects described below. The advantagesdescribed below will be realized and attained by means of the elementsand combinations particularly pointed out in the appended claims. It isto be understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic of an embodiment of nickel multipodformation.

FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D depict temperature and microwavepower profiles for embodiments of nickel multipods formed under constanttemperature mode reactions lasting 3 minutes (FIG. 2A), 4 minutes (FIG.2B), 6 minutes (FIG. 2C), and 10 minutes (FIG. 2D) reactions.

FIG. 3A, FIG. 3C, FIG. 3E, and FIG. 3G depict arm length distributions,and FIG. 3B, FIG. 3D, FIG. 3F, and FIG. 3H depict arm widthdistributions for embodiments of multipods after reactions of 3 minutes(FIG. 3A and FIG. 3B), 4 minutes (FIG. 3C and FIG. 3D), 6 minutes (FIG.3E and FIG. 3F), and 10 minutes (FIG. 3G and FIG. 3H), respectively.FIG. 4A depicts a plot of the length and width of embodiments ofmultipod arms as a function of the number of pulses applied after aninitial pulse.

FIG. 4B depicts a plot of the aspect ratio of the arms of embodiments ofmultipod structures versus the number of pulses after an initial pulse.

FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D, FIG. 5E, and FIG. 5F depicttemperature and power profiles for an embodiment of a pulsed reactionafter 0, 1, 2, 4, 6, and 8 pulses, respectively.

FIG. 6 depicts the overall MW power impingent during embodiments ofreactions operated in pulsed power mode and constant temperature mode.

FIG. 7A, FIG. 7B, FIG. 7C, FIG. 7D, and FIG. 7E are histograms for thearm lengths of products resulting from an embodiment of a pulsed processafter 1 pulse, 2 pulses, 4 pulses, 6 pulses, and 8 pulses, respectively.

FIG. 7F depicts the number of structures having 1 to 6 arms of anembodiment of a pulsed process after 8 pulses.

FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, and FIG. 8E are histograms for armwidth of products resulting from an embodiment of a pulsed process after1 pulse, 2 pulses, 4 pulses, 6 pulses, and 8 pulses, respectively.

FIG. 9A, FIG. 9B, and FIG. 9C depict plots of microwave power andtemperature versus time for three embodiments of methods describedherein.

FIG. 10A, FIG. 10B, FIG. 10C, FIG. 10D, FIG. 10E, and FIG. 10F depict300 K field sweep saturation magnetization curves for embodiments ofmetal multipod nanostructures.

FIG. 11A, FIG. 11B, FIG. 11C, and FIG. 11D depict temperature and powerprofiles of embodiments of reactions carried out in a glass vessel(reaction volume of 3 mL) using either a constant power of 75 W for 4minutes (FIG. 11A) or 4 pulses with different powers (150 W (FIG. 11B),250 W (FIG. 11C), and 300 W (FIG. 11D)).

DETAILED DESCRIPTION

Embodiments of the methods provided herein include the use of pulsed(i.e., cycled) microwave-based heating to achieve the rapid synthesis ofhighly branched, pure phase fcc crystalline nickel multipodnanostructures with a multipod population of >99%. In some embodiments,the multipod structures may be formed in minutes (e.g., 5 to 20 minutes)under ambient conditions in a relatively simple reaction system, anexample of which includes nickel acetylacetonate (Ni(acac)₂), oleylamine(OAm) and oleic acid (OAc).

In some embodiments, controlling the power delivery to a reactionmixture through pulsing allows the growth kinetics of the metallicnanostructures to be controlled, thereby permitting the formation ofmultipods having arms with different aspect ratios. The arm length ofthe metal multipod nanostructures may be proportional to the number ofpulses (i.e., cycles), and the core size may be controlled by continuouspower delivery.

The methods provided herein may permit [1] the rapid synthesis of singlephase fcc Ni multipods using pulsed microwave heating at hightemperature, [2] the synthesis of multipods that are uniform and/orstable at high temperature, [3] the synthesis of magnetic multipodstructures, which may exhibit coercivity that can vary with aspectratio, [4] the use of MW power and/or pulsing to control multipodmorphology, [5] selective microwave heating at the tips ofnanostructures so that the nanostructures' arms elongate relativelyrapidly, or [6] a combination thereof.

Methods

Methods are provided herein for forming metal multipod nanostructures.In some embodiments, the methods include providing a mixture, andcontacting the mixture with microwaves to form the metal multipodnanostructures. The contacting of a mixture with microwaves may includeirradiating the mixture, a microwave reaction vessel in which themixture is disposed, or a combination thereof with the microwaves toheat the mixture.

The mixture may be disposed in a microwave reaction vessel. The phrase“microwave reaction vessel”, as used herein, generally refers to anapparatus that has a reservoir in which a mixture may be disposed, andis formed of one or more materials that permits microwaves to heat themixture. The microwave reaction vessel, for example, may be a glassvessel or a silicon carbide vessel.

In some embodiments, the mixtures include a metal acetylacetonate, areducing agent comprising a C₁-C₃₀ hydrocarbyl-amine, and a carboxylicacid of formula (A)

wherein R is a monovalent C₁-C₃₀ hydrocarbyl.

In some embodiments, the metal acetylacetonate includes a nickel (Ni)acetylacetonate. In some embodiments, the metal acetylacetonate includesa Pt acetylacetonate, a Pd acetylacetonate, a Cu acetylacetonate, a Coacetylacetonate, a Au acetylacetonate, an Fe acetylacetonate, or a Rhacetylacetonate. The metal acetylacetonate may include an acetateprecursor.

In some embodiments, the reducing agent includes a C₁-C₃₀hydrocarbyl-amine. As used herein, the phrase “C₁-C₃₀ hydrocarbyl-amine”generally refers to a monovalent or multivalent C₁-C₃₀ hydrocarbyl thatis substituted with at least one amine. The amine may be a terminalamine. The amine may include a primary amine, a secondary amine,tertiary amine, or a combination thereof. The amine may be a terminalprimary amine.

In some embodiments, the C₁-C₃₀ hydrocarbyl-amine is an unsaturatedC₁-C₃₀ hydrocarbyl-amine. As used herein, the phrase “unsaturated C₁-C₃₀hydrocarbyl-amine” refers to a C₁-C₃₀ hydrocarbyl-amine that includes atleast one non-single bond, such as a double bond.

In some embodiments, the C₁-C₃₀ hydrocarbyl-amine is a C₅-C₃₀hydrocarbyl-amine, a C₁₀-C₃₀ hydrocarbyl-amine, a C₁₅-C₃₀hydrocarbyl-amine, a C₁₅-C₂₅ hydrocarbyl-amine, or a C₁₅-C₂₀hydrocarbyl-amine, which may be saturated or unsaturated. In someembodiments, the C₁-C₃₀ hydrocarbyl-amine is oleylamine.

The metal acetylacetonate and the reducing agent may be present at anymole ratio that is effective to form the metal multipod nanostructures.In some embodiments, the metal acetylacetonate and the reducing agentare present in the mixture at a mole ratio of about 0.3:25 to about3:25. In some embodiments, the metal acetylacetonate and the reducingagent are present in the mixture at a mole ratio of about 0.3:25 toabout 2:25. In some embodiments, the metal acetylacetonate and thereducing agent are present in the mixture at a mole ratio of about0.3:25 to about 1:25. In some embodiments, the metal acetylacetonate andthe reducing agent are present in the mixture at a mole ratio of about0.5:25 to about 1:25. In some embodiments, the metal acetylacetonate andthe reducing agent are present in the mixture at a mole ratio of about0.75:25.

The mixture may include a carboxylic acid of formula (A)

wherein R is a monovalent C₁-C₃₀ hydrocarbyl. In some embodiments, theC₁-C₃₀ hydrocarbyl- is an unsaturated C₁-C₃₀ hydrocarbyl. As usedherein, the phrase “unsaturated C₁-C₃₀ hydrocarbyl” refers to a C₁-C₃₀hydrocarbyl that includes at least one non-single bond, such as a doublebond. In some embodiments, the C₁-C₃₀ hydrocarbyl is a C₅-C₃₀hydrocarbyl, a C₁₀-C₃₀ hydrocarbyl, a C₁₅-C₃₀ hydrocarbyl, a C₁₅-C₂₅hydrocarbyl, or a C₁₅-C₂₀ hydrocarbyl, which may be saturated orunsaturated. In some embodiments, the carboxylic acid of formula (A) isoleic acid.

The reducing agent and the carboxylic acid of formula (A) may be presentin the mixture at any mole ratio that is effective to form the metalmultipod nanostructures. In some embodiments, the reducing agent and thecarboxylic acid of formula (A) are present in the mixture at a moleratio of about 25:2 to about 25:8. In some embodiments, the reducingagent and the carboxylic acid of formula (A) are present in the mixtureat a mole ratio of about 25:3 to about 25:7. In some embodiments, thereducing agent and the carboxylic acid of formula (A) are present in themixture at a mole ratio of about 25:4 to about 25:6. In someembodiments, the reducing agent and the carboxylic acid of formula (A)are present in the mixture at a mole ratio of about 25:5.

The methods herein generally may include contacting a mixture withmicrowaves. The microwaves may be applied in one or more modes describedherein, including a pulsed (i.e., cycled) mode, a constant power mode,and a constant temperature mode.

Pulse Mode (i.e. Cycle Mode): In some embodiments, the contacting of themixture with microwaves includes (i) contacting the mixture with a firstplurality of microwaves effective to heat the mixture to a firsttemperature, (ii) reducing the first temperature of the mixture to asecond temperature, and (iii) contacting the mixture with a secondplurality of microwaves for a time effective to heat the mixture to thefirst temperature. The contacting of the mixture with the secondplurality of microwaves may be referred to herein as a “pulse ofmicrowaves”, a “pulse”, a “cycle”, a “cycle of microwaves”, a “cycle ofmicrowave power”, or the like. The contacting of the mixture with themicrowaves may include contacting the mixture with two or more pulses ofthe microwaves by repeating steps (ii) and (iii) one or more times, forexample, from 1 to 20 times, 1 to 14 times, or 6 to 11 times. Therefore,the contacting of the mixture with the microwaves may include contactingthe mixture with 0 to 21 pulses, 0 to 15 pulses, or 7 to 12 pulses ofthe microwaves.

In some embodiments, the first temperature is about 260° C. to about300° C. In some embodiments, the first temperature is about 280° C.

In some embodiments, the second temperature is about 220° C. to about250° C. In some embodiments, the second temperature is about 240° C.

The reducing of the first temperature of the mixture to a secondtemperature may be achieved by any known active and/or passivetechnique. For example, the reducing of the first temperature of themixture to a second temperature may be achieved by allowing the mixtureto cool in the absence of microwaves. As a further example, the reducingof the first temperature of the mixture to a second temperature mayinclude contacting a vessel and/or mixture with a gas flow, i.e., airflow, which may be provided by a fan or other apparatus.

In some embodiments, the time effective to heat the mixture to the firsttemperature with the second plurality of microwaves is about 1 minute toabout 30 minutes, about 1 minute to about 20 minutes, about 3 minutes toabout 20 minutes, or about 7 minutes to about 12 minutes. The timeeffective to heat the mixture to the first temperature with the secondplurality of microwaves may the same for each pulse, or may differ forone or more of the pulses. For example, each of the 0 to 21 pulses, 0 to15 pulses, or 7 to 12 pulses of the microwaves may be applied for a timeperiod that (i) is independently selected for each pulse, and (ii)selected from about 3 to about 30 minutes, or about 7 to about 12minutes.

Not wishing to be bound by any particular theory, the number of pulsesmay control, at least in part, the arm length of the multipodnanostructures. Therefore, the methods herein may include selecting anumber of pulses effective to achieve a desired arm length.

Not wishing to be bound by any particular theory, the width of one ormore pulses may control, at least in part, the aspect ratio of the arms.The pulse width (i.e., pulse power, e.g., 250 W, 150 W, etc.) can affectthe heating rate of a mixture from a second temperature to a firsttemperature. Therefore, the methods provided herein may includeselecting a width of the one or more pulses effective to impart the armswith a desired aspect ratio.

In some embodiments, the arm length of Ni multipod nanostructures iscontrolled by the number of pulses and/or pulse power applied duringgrowth. In some embodiments, Ni multipod nanostructures with arms thatare 230 nm in length and 51 nm in width (an aspect ratio of 4.5) areprepared by sequential pulsing (9 pulses) in a 2.45 GHz MW single modecavity (300 W) within a total time of 10 minutes. The aspect ratio maybe systematically reduced with fewer pulses. The resulting multipodnanostructures may be highly crystalline metallic single phase fccnickel structures that exhibit magnetic coercivity that varies with theaspect ratio of the arms.

Constant Temperature Mode: In some embodiments, the contacting of amixture with microwaves includes contacting the mixture with a firstplurality of microwaves effective to heat the mixture to a firsttemperature, and contacting the mixture with a second plurality ofmicrowaves effective to maintain the first temperature for a timeeffective to form the metal multipod nanostructures, wherein the secondplurality of microwaves includes microwaves of different wattages. Thewattages of the second plurality of microwaves may be adjusted, manuallyor automatically, to maintain the first temperature of the mixture.

In some embodiments, the first temperature is about 260° C. to about300° C. In some embodiments, the first temperature is about 280° C.

Constant Power Mode: In some embodiments, the contacting of a mixturewith microwaves includes contacting the mixture with a first pluralityof microwaves effective to heat the mixture to a first temperature, andcontacting the mixture with a second plurality of microwaves for a timeeffective to form the metal multipod nanostructures, wherein the secondplurality of microwaves consists of microwaves of the same wattage.Therefore, the wattage of the second plurality of microwaves, in someembodiments, remains unchanged during a process or a portion thereof.For example, the second plurality of microwaves may have a power (e.g.,300 W), and the power is not changed in response to a temperature changeof the mixture. In some embodiments, the power of the first plurality ofmicrowaves and the power of the second plurality of microwaves isidentical. The first plurality of microwaves and second plurality ofmicrowaves may be administered sequentially in a continuous ordiscontinuous manner.

In some embodiments, the first temperature is about 260° C. to about300° C. In some embodiments, the first temperature is about 280° C.

Not wishing to be bound by any particular theory, core nanocrystal sizemay be systematically controlled by a process that combines constantpower mode and pulse mode. The systematic control may be imparted by thelocal electric field enhancement of the impinging microwaveelectromagnetic radiation of the overgrowth (111) tips in fcc metalsthrough a “lightning rod effect.”

Not wishing to be bound by any particular theory, it is believed thatthe interaction of high-power (and short time) MW pulses with thegrowing Ni nanoparticle may produce local tip heating, which can lead to(111) growth of arms. In constant power mode, the system may reachequilibrium and the growth of the (111) face may be in competition withnanocrystal reconstruction to minimize surface energy. Reactions usinglow-power pulses (which can result in longer times to reach a firsttemperature from a second temperature) or constant continuous power maylead to nanostructures without long arm lengths.

Generally, the microwaves used in the foregoing modes may have anyfrequency effective to form multipod nanostructures. In someembodiments, the microwaves have a frequency of at least 2 GHz or atleast 2.45 GHz. In some embodiments, the microwaves have a frequency of2.45 GHz.

Metal Multipod Nanostructures

The metal multipod nanostructures produced according to embodiments ofthe methods herein may have 1 to 8 arms. In some embodiments, the one ormore metal multipod nanostructures have 1 to 8 arms, and an average of 4to 6 arms. In some embodiments, the methods described herein produce aplurality of metal multipod nanostructures, of which at least 95%, byweight, of the metal multipod nanostructures have 1 to 8 arms, anaverage of 4 to 6 arms, or a combination thereof.

The metal multipod nanostructures produced according to embodiments ofthe methods described herein may include arms having an average aspectratio of about 1 to about 5, about 2 to about 5, about 3 to about 5, orabout 4 to about 5. In some embodiments, the metal multipodnanostructures have arms having an average aspect ratio about 4.5. Forexample, Ni multipods produced according to embodiments of the methodsdescribed herein have arms that are about 230 nm in length and about 51nm in width, which imparts the arms with an aspect ratio of about 4.5.

The metal multipod nanostructures produced according to embodiment ofthe methods described herein may have arms having an average length ofabout 0.1 nm to about 300 nm, about 100 nm to about 300 nm, or about 200nm to about 300 nm.

The phrase “C₁-C₃₀ hydrocarbyl,” and the like, as used herein, generallyrefer to aliphatic, aryl, or arylalkyl groups containing 1 to 30 carbonatoms. Examples of aliphatic groups, in each instance, include, but arenot limited to, an alkyl group, a cycloalkyl group, an alkenyl group, acycloalkenyl group, an alkynyl group, an alkadienyl group, a cyclicgroup, and the like, and includes all substituted, unsubstituted,branched, and linear analogs or derivatives thereof, in each instancehaving 1 to 30 carbon atoms. Examples of alkyl groups include, but arenot limited to, methyl, ethyl, propyl, isopropyl, n-butyl, t-butyl,isobutyl, pentyl, hexyl, isohexyl, heptyl, 4,4-dimethylpentyl, octyl,2,2,4-trimethylpentyl, nonyl, decyl, undecyl and dodecyl. Cycloalkylmoieties may be monocyclic or multicyclic, and examples includecyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and adamantyl.Additional examples of alkyl moieties have linear, branched and/orcyclic portions (e.g., 1-ethyl-4-methyl-cyclohexyl). Representativealkenyl moieties include vinyl, allyl, 1-butenyl, 2-butenyl,isobutylenyl, 1-pentenyl, 2-pentenyl, 3-methyl-l-butenyl,2-methyl-2-butenyl, 2,3-dimethyl-2-butenyl, 1-hexenyl, 2-hexenyl,3-hexenyl, 1-heptenyl, 2-heptenyl, 3-heptenyl, 1-octenyl, 2-octenyl,3-octenyl, 1-nonenyl, 2-nonenyl, 3-nonenyl, 1-decenyl, 2-decenyl and3-decenyl. Representative alkynyl moieties include acetylenyl, propynyl,1-butynyl, 2-butynyl, 1-pentynyl, 2-pentynyl, 3-methyl-1-butynyl,4-pentynyl, 1-hexynyl, 2-hexynyl, 5-hexynyl, 1-heptynyl, 2-heptynyl,6-heptynyl, 1-octynyl, 2-octynyl, 7-octynyl, 1-nonynyl, 2-nonynyl,8-nonynyl, 1-decynyl, 2-decynyl and 9-decynyl. Examples of aryl orarylalkyl moieties include, but are not limited to, anthracenyl,azulenyl, biphenyl, fluorenyl, indan, indenyl, naphthyl, phenanthrenyl,phenyl, 1,2,3,4-tetrahydro-naphthalene, tolyl, xylyl, mesityl, benzyl,and the like, including any heteroatom substituted derivative thereof.

Unless otherwise indicated, the term “substituted,” when used todescribe a chemical structure or moiety, refers to a derivative of thatstructure or moiety wherein one or more of its hydrogen atoms issubstituted with a chemical moiety or functional group such as alcohol,alkoxy, alkanoyloxy, alkoxycarbonyl, alkenyl, alkyl (e.g., methyl,ethyl, propyl, t-butyl), alkynyl, alkylcarbonyloxy (—OC(O)alkyl), amide(—C(O)NH-alkyl- or -alkylNHC(O)alkyl), tertiary amine (such asalkylamino, arylamino, arylalkylamino), aryl, aryloxy, azo, carbamoyl(—NHC(O)O-alkyl- or —OC(O)NH-alkyl), carbamyl (e.g., CONH₂, as well asCONH-alkyl, CONH-aryl, and CONH-arylalkyl), carboxyl, carboxylic acid,cyano, ester, ether (e.g., methoxy, ethoxy), halo, haloalkyl (e.g.,—CCl₃, —CF₃, —C(CF₃)₃), heteroalkyl, isocyanate, isothiocyanate,nitrile, nitro, phosphodiester, sulfide, sulfonamido (e.g., SO₂NH₂),sulfone, sulfonyl (including alkylsulfonyl, arylsulfonyl andarylalkylsulfonyl), sulfoxide, thiol (e.g., sulfhydryl, thioether) orurea (—NHCONH-alkyl-).

In the descriptions provided herein, the terms “includes,” “is,”“containing,” “having,” and “comprises” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to.” When structures or methods are claimed or described interms of “comprising” various components or processing features, thestructures and methods can also “consist essentially of” or “consist of”the various components or processing features, unless stated otherwise.

The terms “a,” “an,” and “the” are intended to include pluralalternatives, e.g., at least one. For instance, the disclosure of “amultipod,” “a C₁-C₃₀ hydrocarbyl-amine,” and the like, is meant toencompass one, or mixtures or combinations of more than one multipod,C₁-C₃₀ hydrocarbyl-amine, and the like, unless otherwise specified.

The processes described herein may be carried out or performed in anysuitable order as desired in various implementations. Additionally, incertain implementations, at least a portion of the processes may becarried out in parallel. Furthermore, in certain implementations, lessthan or more than the processes described may be performed.

Many modifications and other implementations of the disclosure set forthherein will be apparent having the benefit of the teachings presented inthe foregoing descriptions and the associated drawings. Therefore, it isto be understood that the disclosure is not to be limited to thespecific implementations disclosed and that modifications and otherimplementations are intended to be included within the scope of theappended claims.

EXAMPLES

The present invention is further illustrated by the following examples,which are not to be construed in any way as imposing limitations uponthe scope thereof. On the contrary, it is to be clearly understood thatresort may be had to various other aspects, embodiments, modifications,and equivalents thereof which, after reading the description herein, maysuggest themselves to one of ordinary skill in the art without departingfrom the spirit of the present invention or the scope of the appendedclaims. Thus, other aspects of this invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein.

Example 1 Microwave Synthesis of Nickel Multipods

Size-tuneable nickel multipod structures were synthesized in thisexample using a pulsed microwave heating approach. The approach of thisexample provided control over growth and produced quality metallicnickel nanostructures with tight size distributions.

The approach of this example used only three reactants. The processes ofthis example were carried out under ambient conditions, and werecompleted within minutes with the use of a microwave reactor. Synthesistimes for even the largest multipods of this example took less than 15minutes with the pulsed microwave approach of this example.

The microwave power influenced the structure of the multipods of thisexample. The higher power pulses produced highly branched structures andlonger arm lengths.

The kinetics of the reaction were believed to be important in successfulsynthesis of multipods that have not been easily achievable throughconvective heating means.

Results from this example suggested that a temperature differencebetween (i) the multipod tips and (ii) the rest of the nanoparticle wasachieved through selective heating of the tips during the pulse step ofthe reaction of this example, and this temperature difference wasbelieved to permit the arms to grow rapidly along the tip.

Particle analysis by pXRD, TEM, and SQUID revealed that the processes ofthis example produced pure fcc phase Ni structures with arms that grewalong the <111> direction, and that exhibited magnetic coercivitybehaviour at 300K. This behavior showed a positive correlation with theaspect ratio of the multipod arms.

The pulsed heating approach of this example can be applied to othermaterial types, and was believed to demonstrate the role ofmicrowave-matter interactions in the synthesis of materials usingmicrowave-based heating.

The following procedure was used in this example: 0.75 mmol of nickelacetylacetonate, 25 mmol of oleylamine and 5 mmol of oleic acid wereadded to a G30 (30 mL) microwave vessel (Anton Paar USA Inc., USA) andsealed with a silicone septa and snap cap. The 9 mL of solution wasdegassed under vacuum with stirring at a temperature of about 100° C.using a water bath until no more bubbling was observed.

The solution turned blue upon completion. The pierced silicone septumwas replaced with a polytetrafluoroethylene (PTFE) septum under ambientconditions, and no stir bar or inert atmosphere was used or introduced,respectively, in the reaction vessel. The vessel was then placed into anMONOWAVE™ 300 microwave system, and heated at a constant power of 300 Wto 280° C. with the stir setting at 600 rpm.

Upon achieving a temperature of 280° C., one of several operationalmodes were used: (1) constant temperature mode (280° C.) (wherein theinstrument maintained the temperature by adjusting the powerautomatically), and (2) constant power mode for a specific time (whereinthe temperature of the reaction depended on the heating rate imparted bythe delivery of continuous power), or (3) a custom setup wherein theinstrument was programmed to cycle through cooling the reaction to acertain temperature (240° C. in this case) followed by heating with adesired constant power (e.g., 300 W, 250 W, 150 W) up to a certaintemperature (280° C. in this example), which provided the ability topulse the microwave for a number of cycles before the final automaticcool down to 55° C.

For experiments using an 10 mL silicon carbide vessel (SiC10) or a 10 mLglass vessel (G10), 3 mL of the solution was used in the reaction ofthis example.

For comparison purposes, a convective reaction using a round bottomflask was carried out to evaluate the proposed growth model.

FIG. 1 depicts a proposed growth mechanism for the Ni multipods grownunder the pulsed MW conditions versus the continuous power conditions ofthis example. Not wishing to be bound by any particular theory, it isbelieved that the growth of Ni occurred via a 2-step autocatalyticprocess, wherein the slow step involved Ni(acac)₂ reduction byoleylamine to form a nuclei, followed by fast surface mediated growth inwhich the nickel nuclei is capped by oleylamine and oleic acid. Ammoniagas produced by the reduction mechanism of Ni(acac)₂ by oleylamine mightalso have acted as a small molecule with selective surface binding. Therates of growth were believed to be controlled by facet energy with the(111) facet being energetically favored. During the microwave pulsestep, selective-heating of (111) tips on nanoparticle corners couldpermit faster autocatalytic growth at the tips, thereby leading toelongation of the (111) facet and forming the observed multipodstructures, which are schematically depicted at FIG. 1. Under continuouspower, the particle likely reached thermal equilibrium, thereby leadingto facet reorganization.

The degree of experimental control achieved by pulsing versus continuousMW power was determined, at least in part, by electron microscopyimages, which were collected for the products of this example. Theimages showed that initial nucleation and growth resulted in cubicstructures that evolved towards (111) overgrowth when maintained undercontinuous power, but lead to high aspect ratio multipod morphologiesunder pulsed conditions.

The evolution from cubic to overgrowth to high aspect ratio multipodswas analysed as a function of time (number of pulses) to produce amechanism for growth that distinguished (i) autocatalytic growthobserved in MW reactors for Ni under continuous heating from (ii) theonset of MW enhanced (111) facet growth under pulsed MW conditions.

Evolution of multipods under constant temperature mode for MW heating(variable power): The influence of time on multipod formation wasevaluated under constant temperature conditions where the MW power wasfluctuated to maintain temperature over the course of the reaction.

The reactions were carried out by heating a solution of Ni(acac)₂dissolved in a 5:1 (V:V) ratio of oleylamine to oleic acid to 280° C.using an initial incident power of 300 W to reach 280° C., and allowingthe power to fluctuate to maintain the reaction temperature. Thereaction temperatures and microwave power profiles used during this setof reactions are shown at FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D.

FIG. 2A, FIG. 2B, FIG. 2C, and FIG. 2D depict temperature and microwavepower profiles for the nickel multipods under constant temperature modefor 3 minute (FIG. 2A), 4 minute (FIG. 2B), 6 minute (FIG. 2C), and 10minute (FIG. 2D) reactions.

As depicted at FIG. 2A, the power pulses after 2 minutes dropped from300 W to 0 W, with continuous power fluctuations at about 50 W to about100 W during the reaction to maintain reaction temperature. Reactionscarried out for 3 minutes (i.e., 1 minute beyond the pulse event),during the largest variance in power, produced irregularly-shapednanoparticles, which exhibited overgrowth, but wide dispersities in size(about 20 nm to about 100 nm).

After 4 minutes (i.e., 2 minutes beyond the pulse event), thenanoparticles grew larger and exhibited clear multipod morphology (e.g.,an arm aspect ratio of 1.4) with tapering arms. From 6 minutes to 10minutes, the particles exhibited variability in the multipod morphology,with large sphere-like cores with longer arms of uniform width androunded ends extending out from the core (arm aspect ratio of about 1.6to about 1.4). The stepped power experiment produced non-uniform shapedNi, which was consistent with the 6-10 minute reactions.

The average arm length, arm width, and aspect ratios were extracted fromTEM images, and are reported at the following table (Table 1).Distributions extracted from the TEM images are provided at FIG. 3A,FIG. 3B, FIG. 3C, FIG. 3D, FIG. 3E, FIG. 3F, FIG. 3G, and FIG. 3H. FIG.3A, FIG. 3C, FIG. 3E, and FIC. CG depict arm length distributions, andFIG. 3B, FIG. 3D, FIG. 3F, and FIG. 3H depict arm width distributionsfor the multipods of this example after reactions of 3 minutes, 4minutes, 6 minutes, and 10 minutes, respectively.

TABLE 1 Multipod Arm Length, Width, and Aspect Ratio with RespectiveStandard Deviations for Various Reactions Using Constant TemperatureMode (FIG. 2A-FIG. 2D*) Arm Arm Aspect Length Width Ratio Reaction ArmStandard Arm Standard Relative Time Length Dev. Width Dev. AspectStandard (minutes) (nm) (nm) (nm) (nm) Ratio Deviation 3 16 7.5 14 5.11.1 0.59 4 55 19 38 10 1.4 0.44 6 77 26 50 13 1.6 0.43 10 81 31 58 101.4 0.42 *The reaction for constant power mode (75 W, 6 minutes)produced sphere-like nanoparticles with a diameter of 72 ± 12 nm.

Arm length was determined as the length from tip to base of the arm, andwidth was measured across the center region of the multipod arm.Analysis of the 10 minute multipod (FIG. 2D, FIG. 3G, and FIG. 3H)confirmed that the isolated samples were pure fcc crystal structuresaccording to pXRD.

No clear correlation was observed between the length of reaction at 280°C. (constant temperature) and the appearance of multipod arms. Thevariance was believed to be attributed to the lack of reproducibility inthe power pulses that arose under constant temperature mode in a MW,even with, on average, constant power with respect to time.

High resolution TEM of the arms indicated that the arms grew along the<111> direction with a d-spacing of 0.20 nm, with no visible glide planeerrors. Specifically, a HRTEM image of the arms in the 4 minute reactionshowed that the arms grew along the <111> direction with a d-spacing of0.20 nm, with no visible glide plane errors. In the 10 minute reaction,the overgrowths on the 10 minute Ni nanoparticle also conformed to the<111> plan with a d-spacing of 0.20 nm. Powder X-ray diffraction (pXRD)patterns of the 10 minute reaction confirmed the d-spacing assignmentsto fcc Ni.

The lack of a clear power or time dependence on growth behavior usingconstant temperature was could likely be attributed to the largefluctuation in the MW power cycle profiles. To assess the effect of MWinfluence, controlled convective reactions were carried out underreaction conditions identical to those of the constant temperaturestudies of this example (280° C., 5:1 OAm:OAc, 0.08 M Ni(acac)₂) for 4minutes in a preheated aluminum block and 1 hour using a round-bottomflask with a heating mantle (see Example 2).

In the convective reactions, the NiNPs appeared as sphericalnanocrystals in SEM images. The formation of overgrowth and multipodarms was not observed under convective conditions. The lack ofovergrowth under the experimental conditions was consistent withprevious reports indicating that high temperature reactions may lead tospherical nanocrystals.

The shapes of the NiNPs grown under convective conditions compared toNiNPs grown in a MW were remarkably different. The NiNPS grown in a MWexhibited overgrowth leading to multipod structures. Inspection of thepower vs. time graphs suggested that the multipod formation in the MW ofthis example was potentially due to the presence of pulsing.

Evolution of multipods under pulsed, i.e., cycled, microwave power(variable temperature and reaction time): The influence of pulsing onmultipod formation was evaluated under variable temperature and time toevaluate whether pulsing of the impingent MW field directly correlatedto the length of the arm observed in the MW reactions.

The evolution of nickel multipod structures synthesized using the pulsedpower mode of this example with an increasing number of pulses wasobserved via TEM images at 0 pulses, 1 pulse, 2 pulses, 4 pulses, 6pulses, and 8 pulses. High resolution TEM images showed the crystallinelattices of the multipod arms. FIG. 4A depicts a plot of the length andwidth of the multipod arms as a function of the number of pulses appliedafter the initial pulse, and FIG. 4B depicts a plot of the aspect ratioof arms versus the number of pulses after the initial pulse.

The reactions of this particular group were carried out under reactionconditions that were identical to those of the constant temperaturestudy; however, the MW was pulsed at 300 W in a controlled fashion, andthe temperature maximum was set to 280° C. The temperature was allowedto drop to 240° C. between pulses.

The temperature and power profiles for the pulsed reactions are providedat FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D, FIG. 5E, and FIG. 5F. The averagetemperature of the reaction was about 260° C. The overall MW powerimpingent of the samples was higher than the power impingent of thesamples during constant temperature mode, as depicted at FIG. 6. Forexample, an 8 pulse reaction requiring 4.5 minutes of reaction timeimparted a 38% higher MW power impingent on the reactants compared tothe reaction at constant reaction temperature (280° C.) for the samereaction time.

At zero pulse (FIG. 5A), cube shaped nanoparticles of about 26 nm indiameter were observed. With an increasing number of pulses (FIG.5B-FIG. 5F), anisotropic multipod structures were observed, wherein thearm length and aspect ratio increased with the number of pulses.

The isolated materials were fcc, based on powder X-ray diffraction data.The multipod structures had crystal structures that matched the fcc Nireference pattern PDF #01-087-0712. As demonstrated by the plots of theaverage length of the arms and widths (FIG. 4A), as well as the aspectratio (length/width) (FIG. 4B), the increase in arm length, arm widthand aspect ratio appeared to be substantially linear.

FIG. 7A, FIG. 7B, FIG. 7C, FIG. 7D, and FIG. 7E are histograms for armlength after 1 pulse, 2 pulses, 4 pulses, 6 pulses, and 8 pulses,respectively. FIG. 7F depicts the number of structures having 1 to 6arms after 8 pulses. FIG. 8A, FIG. 8B, FIG. 8C, FIG. 8D, and FIG. 8E arehistograms for arm width after 1 pulse, 2 pulses, 4 pulses, 6 pulses,and 8 pulses, respectively.

An analysis of an SEM image of the product of the 8 pulse reactionindicated >95% multipod formation (within 2σ) with an average of fourarms per multipod. The distribution, as depicted at FIG. 7F, was aGaussian distribution. The average length of the arms was 228±33 nm, andthe average width was about 51±8.2 nm, which resulted in an aspect ratioof 4.5.

The multipod arms exhibited no glide plane errors, and grew as <111>extentions. High resolution TEM imaging of the 4 cycle and 8 cyclesamples revealed that the arms were single crystal along the <111>direction, with d-spacing of 0.2 nm. A high resolution TEM image of someinitially formed Ni cores (0 cycle reaction) and a region near thecore-arm interface for the 8 cycle reaction had a defect-freesingle-crystalline nature.

The increase in the aspect ratio for the <111> art is depicted for the10 s MW-on cycles at FIG. 4B. The aspect ratio appeared to be linearwith respect to the number of cycles. At FIG. 9A, FIG. 9B, and FIG. 9C,the impact of shorter MW heating cycles is depicted. Comparison ofnearly equivalent heating times for a single 10 s cycle, two 5 s cycles,and three 3 s cycles following the initial 13 s MW irradiation eventconfirmed the control of aspect ratio. The arm length and width wasextracted by analysis of 300 arms in SEM images. For the 10 s cycle(FIG. 9A), the arm length was 178±37.9 nm and the arm width was57.6±7.87 nm (aspect ratio 3.1); for the two 5 s MW-on cycles (FIG. 9B),the lenth was 246±44.4 nm and the arm width was 42.2±10.1 nm (aspectratio 5.8); and for the three 3 s MW-on cycles (FIG. 9C), the length was277±49.6 nm and the arm width was 41.1±10.5 nm (aspect ratio 6.8). Fromthe cycling study, it was demonstrated that for the shorter MW-on cycletimes, thinner arms were observed.

The multipods of the pulsed experiments of this example were compared tothose that resulted from the constant temperature experiments of thisexample, and the comparison revealed that under pulse conditions theuniformity of the multipods was significantly better, with a highermultipod density. The observed multipod density was believed to be thehighest ratio reported to date, with >99% for processes of four or morepulses. The arms exhibited no glide plane errors and grew as (111)extensions.

The role of pulsing a MW to control multipod formation was furtherdemonstrated in this example by using constant power followed by pulsingat the end of the reaction. The results of a 25 minute reaction carriedout at constant temperature for 15 minutes, followed by >8 pulses led tomultipod structures observed in the TEM image with >80% multipodformation, although other morphologies were present. The experiment ledto larger core sizes (with an average arm length of 201±64 nm and awidth of 59±13 (aspect ratio of 3.4)). The number of arms, on average,was four. While the results were not as uniform as the pulse-onlyexperiments of this example, the observation of multipod growthconfirmed the role of pulsing on multipod isolation.

Not wishing to be bound by any particular theory, it was believed thatthe growth of nickel nanoparticles of this example in a microwavefollowed an autocatalytic 2-step Finke-Watsky mechanism, wherein thefirst step involved a slow reduction step of the nickel precursor insolution by the weak reducing agent, oleylamine, and a fast reductionstep of the nickel precursor at the surface of the growing nickelnanoparticle which acted as a catalyst. The presence of oleylamine andoleic acid (which both acted as capping ligands involved in shapingnanostructures), binded selectively to different facets of the evolvingnickel nuclei based on differences in binding energies to differentcrystal facets. It also was believed that the surface atom rearrangementon crystals could control the shape of the final nanoparticle, and itwas believed that the surface atom diffusion on the nickel nucleic wasminimal and therefore gave rise to anisotropic structures.

Effect of cycle power on multipod evolution: Although it was desirableto measure the tip versus core of the Ni multipods of this exampleduring the MW cycle to support and/or demonstrate the “lightning rode”mechanism, it was not practical to do so on the scale of thenanoparticles.

The microwave apparatus measured vessel temperature and not thenanoparticles' temperature directly. In order to demonstrate that growthbehaviour was dependent on MW-on cycling leading to tip heating, aseries of experiments were carried out wherein the influence of MW poweron the growth behaviour was evaluated. It was believed that the highcycle powers would lead to hotter tips, even though the average reactiontemperature was nearly constant. However, the time to achievetemperature was lengthened at lower power, which should have causenanoparticle temperature equilibration. For the reasons of this example,a 10 mL reaction vessel was used, which likely impacted the MW energyabsorbed and/or the growth rates.

SEM images revealed that the lowering of the power of the 4 cyclereaction to 150 W resulted a greatly reduced population of multipods,and the formation of mostly spherical nanoparticles (diameter=52.4±13.9nm). This result differed from the result achieved by the 300 W 4 cyclereaction, which produced well-defined multi-pods (arm length=73.6±27.1,arm width=33.5±5.6 nm).

Two other experiments also were carried out, wherein the sequentialcycle power of the 4 cycles was either increased or decreased by 50 W.When the cycle power was sequentially decreased from 300 to 150 W in a 4cycle reaction, the nanostructures formed, according to SEM images, weremore rounded, with very little overgrowth (arm length=22.2±6.7 nm, armwidth=13.9±3.4 nm). In the other case, wherein the MW power ofsequential cycles was increased from 150 to 300 W, multipods with longerarms were formed, according to SEM images (arm length=55.9±18.5 nm, armwidth=28.0±5.2 nm).

The power dependent growth behaviour could be ascribed to a lowertemperature differential between the core and the tips when the cyclepower was sequentially decreased, which could have caused more uniformnanoparticle surface heating and growth. When, however, the cycle powerwas increased sequentially, the initially formed overgrowths continuedto be selectively heated compared to the rest of the nanoparticlesurface with subsequent high-power short-time cycles as previouslydescribed.

Role of ligands on multipod generation under constant temperature andcycled MW power modes: As a further check on the MW-on cycle-dependentgrowth, the influence of the ligand was investigated. A set ofexperiments was carried out under both constant temperature (8 minutes)and cycled MW power (4 cycles of 300 W), wherein the ratio of the OAm(reducing agent and capping agent) and OAc (capping agent) was varied.Also investigated was the effect of nanoparticle growth when the primaryamine (OAm) was substituted with a tertiary amine, trioctylamine.

TEM and SEM images revealed that the reactions carried out at 5:1OAm:OAc in the 10 mL reaction vessel produced multipod structures. Theformation of the multipod was likely to be enhanced by the presence ofOAc, due to its ligand-directing ability. Reactions carried out at 1:5or 0:1 OAm:OAc did not yield product, likely due to too little reducingagent, which was consistent with the requirement of an amine to act as areducing agent to initially cause a reduction of Ni(acac)₂ to initiateNi growth.

When the MW experiments were carried out at 1:0 OAm:OAc,spherical-shaped nanoparticles were formed, which was consistent withthe understanding that OAc directs, at least in part, nanoparticleshape. The isolated Ni spheres from the constant temperature conditionshad a diameter of 74.5±13.0 nm, and following a reaction with 12 cyclesat 300 W, the nanoparticle diameter was 137±37 nm.

The Ni nanoparticles formed using a 5:1 v/v TOA:OAc ratio alsoeliminated multipod growth under constant temperature and cycled MWpower modes. Under the constant temperature mode, sphericalnanoparticles with a diameter of 4.63±1.62 nm were isolated, whereas thecycled MW power mode produced large faceted nanoparticles with diametersof 173±70 nm. The loss of multipod structure may be been due to a lossof packing order at the surface, possible due to the presence of boundTOA.

Analysis of the magnetic and thermal stability properties of Nimultipods: The multipod structures with high anisotropic structures mayhave applications in a range of technical fields, including magnetismand catalysis. The magnetic characterization and thermal stability ofselected multipods, therefore, were analysed. The data are provided atFIG. 10A, FIG. 10B,

FIG. 10C, FIG. 10D, FIG. 10E, and FIG. 10F, which depict 300 K fieldsweep saturation magnetization curves that show coercivity for multipodswith arms having an aspect ratio of 2.17 (coercivity 215 Oe) (FIG. 10Aand FIG. 10D), multipods with arms having an aspect ratio of 2.42(coercivity 250 Oe) (FIG. 10B and FIG. 10E), and multipods of aspectratio 4.47 (coercivity 283 Oe) synthesized using a cycle power of 300 W(10 s) (FIG. 10C and FIG. 10F).

The results from the magnetic field-sweep studies carried out at 300 Kshowed that the anisotropic nature of the multipod arms influenced thecoercivity of the materials, while maintaining a high saturationmagnetization value. The coercivity increased from 215 Oe (arm aspectratio of 2.17) to 250 Oe (arm aspect ratio of 2.42) and finally 283 Oe(arm aspect ratio of 4.47) as the aspect ratio of the multipod armsincreased. In comparison, the spherical Ni fcc nanoparticles of 55 nmdiameter that were synthesized exhibited a coercivity of only 109 Oe at300 K. The values of coercivity at 300 K for Ni multipods from the 8cycle reaction were greater than that of many known Ni nanostructures.

In order to probe the structural stability of the multipods at highertemperatures, which are typically used for catalytic studies, a seriesof experiments was carried out wherein the multipod structures from the300 W 8 cycle reaction were thermally heated at 200, 400, and 600° C.for 30 minutes in a thermogravimetric analysis (TGA) instrument andimaged by SEM post-treatment. The SEM images for the multipods pre- andpost-thermal treatment showed that the multipod morphology wasmaintained even at 400° C. without any observation of reconstruction tospherical morphology.

At 600° C., the particles started to fuse with each other as ligand losshad taken place, and melting alongside surface reconstruction of themultipods seemed to occur at this temperature.

The following materials were used in the examples herein: 99%, nickelacetylacetonate hydrate (Ni(acac)₂.xH₂O), oleic acid (OAc), oleylamine(OAm) technical grade 70%, toluene, methanol (MeOH), acetone, andchloroform were purchased from Sigma Aldrich. The materials were usedwithout further purification.

Example 2 Synthesis of Nickel Nanoparticles by Convective Heating

To further demonstrate that MW pulsing enhanced overgrowth and (111)facet elongation, convective reactions were carried out under identicalreaction conditions (280° C., 5:1 OAm:OAc, 0.08 M Ni(acac)₂) for 4minutes. In the convective reactions, the NiNPs appeared as sphericalfcc nanocrystals, wherein the observed size increased with reactiontime.

The formation of overgrowth and multipod arms was not observed underconvective conditions. The lack of overgrowth under the experimentalconditions was consistent with previous reports, whereing hightemperature reactions led to spherical Ni nanocrystals.

The shapes of the Ni NPS grown under convective conditions compared toNi NPs grown in a MW were remarkably different. The NiNPS grown in a MWexhibited overgrowth leading to multipod structures. Inspection of thepower versus time graphs suggested that multipod formation in the MW waspotentially due to the presence of pulsing.

Heating mantle method: 18 mL of the solution used to synthesize nickelmultipods listed previously was added to a round bottom flask that washeated to 280° C. from room temperature (4° C./min) and held at 280° C.for 1 hr using a heating mantle after degassing under vacuum at 100° C.for 30 min. The reaction was cooled down quickly to room temperatureusing a blower and the nanoparticles were cleaned up.

Aluminum block method: A G10 vessel or SiC10 vessel containing 3 mL ofsolution and fitted with a septa and cap was placed in an aluminum blockand degas sed at 100° C. under vacuum for 30 minutes before heating to280° C. (4° C./min), and being maintained for 4 minutes at thattemperature prior to removal of the vessel from the block and coolingthe vessel to room temperature using a blower.

Hot injection method: A well-insulated G10 vessel fitted with septa andcap was placed in an aluminum block and degassed at 100° C. under vacuumbefore being heated to 290° C. (4° C./minute) and stabilized at thattemperature. A 3 mL solution that was degassed at 100° C. was injectedrapidly into the pre-heated G10 vessel at 280° C. and maintained for 10minutes prior to its removal and cooling to room temperature using ablower. The temperature of the reaction was observed to drop to 280° C.upon injection.

Example 3 Clean-Up and Analytical Techniques

Clean up of nanoparticles: Post-reaction, a small amount of toluene wasadded to the microwave vessel containing the nanoparticles, and thecontents of the vessel were then sonicated.

The particles were pulled to the side of the vessel using a smallmagnet, and the supernatant solution was discarded. The particles werebrought up in toluene and magnetically separated again. The sameprocedure was repeated using methanol instead of toluene.

This process was repeated for 2 cycles of toluene and methanol beforethe particles were dried under vacuum for characterization.

In the case of smaller particles that were not separated magnetically,the nickel nanoparticles were precipitated by the addition of 5 mLtoluene followed by 15 mL of methanol. The resulting solution wascentrifuged for 5 minutes using a centrifuge tube. After removing thesupernatant, the pellet was redispersed in toluene. To precipitate theNiNPs, excess methanol was added, followed by isolation throughcentrifugation before drying under vacuum.

Transmission Electron Microscopy (TEM): Nanoparticle samples weredrop-cast, from a toluene dispersion, onto 300 mesh carbon coated coppergrids, and left to dry under vacuum overnight. The TEM images wererecorded using a JEM-ARM200cF electron microscope at 200 kV accelerationvoltage.

Scanning Electron Microscopy (SEM): SEM imaging was performed onaluminum mounts with nanoparticles drop-casted directly and allowed todry. SEM imaging was performed on a FEI NOVA NANOSEM™ 400 scanningelectron microscope operating at 20 kV with a spot size of 4.0. Theimages were collected with an Everhart-Thornley detector (ETD). An INCA™X-Sight energy dispersive spectroscopy (Oxford Instruments, USA) (EDS)detector was used for EDS analysis.

Thermogravimetric Analysis (TGA): TGA was performed on a Q50thermogravimetric analyzer (TA Instruments, USA). The samples wereheated at a rate of 10° C./minute from room temperature to 105° C. andheld for 10 minutes before continuing to ramp up the temperature at 10°C./min to 800° C. Measurements were performed under argon to preventfurther oxidation.

Magnetic Measurements: Magnetic properties were studied with asuperconducting quantum interference device (SQUID) magnetometer,MPMS-XL (Quantum Design). Field-dependent magnetization was measured at300 K, with the applied field varying from 0 T to 1.5 T and back.

Magnetic properties of different pulse number multipods: Hysteresisloops at 300K were plotted.

Field sweep studies at 300 K from −1.5 T to 1.5 T were carried out onsome of the multipod structures. The saturation magnetization of themultipods did not differ significantly, but a clear difference incoercivity was observed between the samples, with the coercivityincreasing as the aspect ratio increases.

The coercivity at 300 K was found to be 200 Oe, 250 Oe, and 275 Oe forthe multipods with arms of aspect ratios 1, 2.4 and 4.5 respectively.This value was quite high for nickel nanoparticles at 300K and likelywould be significantly increased at lower temperatures.

This was believed to suggest that shape anisotropy played a role in themagnetic properties of the multipods, and could be tuned by changing theaspect ratios of the arms (Cowburn, R. P. “Property variation with shapein magnetic nanoelements.” Journal of Physics D: Applied Physics 33.1(2000): R1). Coercivity in nickel has been reported to range between0-290 Oe.

Powder X-Ray Diffraction (pXRD): The pXRD patterns for Ni nanoparticleswere acquired on a Rigaku Ultima III diffractometer equipped with aCu—Kα source. Data was collected at room temperature, in the 20 range of10-84°.

Pulse power effect: In order to investigate the effect of the microwavepulse width (and directly pulse power), which can affect the heatingrate between 240° C. and 280° C., a number of reactions were carried outwherein the power of the pulse was reduced to 250 W and 150 W. Thetemperature and power profiles for these reactions are provided at FIG.11A-FIG. 11D. FIG. 11A-FIG. 11D depict the temperature and powerprofiles of different reactions carried out in a glass vessel (reactionvolume of 3 mL) using either constant power of 75 W for 4 minutes (FIG.11A) or 4 pulses with different powers (150 W (FIG. 11B), 250 W (FIG.11C), and 300 W (FIG. 11D)). The average pulse time was 27 s (150 W), 15s (250 W) and 13 s (300 W) to transition from 240° C. to 280° C., and 10s to cool from 280° C. to 240° C. (in all cases). As the power of thepulse increased, the pulse width and overall reaction time decreased.Each reaction was initially brought to 280° C. using 300 W constantpower to allow a similar nucleation process.

The nanoparticles from reactions using 75 W for 4 minutes and 4 pulsesof 150 W did not show significant numbers of nanoparticles with arms,and the nanoparticles appeared to be more spherical or oblong. Thenanoparticles synthesized with 4 pulses of 250 W were observed to besmaller, with shorter arms compared to those made using 4 pulses of 300W. This set of experiments indicated the flux of microwave energy wasmore important, at least in this example, for producing longer armmultipod structures than the total power delivered or total reactiontime above 240° C.

Vessel effect: To elucidate the role of microwave heating, an equivalent4-pulse reaction was carried out in a G10 and a SiC10 vessel.

The SiC vessel had a higher thermal conductivity and microwaveabsorption cross-section compared to the glass vessel, which allowed itto heat/cool faster and reduce the microwave energy penetration.

The nanoparticles from the SiC vessel showed a mixture of morphologies,including spherical particles, multipods with short arms, and a numberof large flat sheet-like structures. The particles from reactionperformed in the glass vessel showed more uniform multipod structures.

A possible reason for the poor size and shape control in the SiC vesselmight have been a thermal gradient created in the SiC vessel from thehot inner surface of the vessel to a cooler region in the centre of thevessel, due, at least in part, to the high thermal conductivity of SiCand lower microwave energy penetration into the solution.

Attempts to synthesize the multipods using convective heating using around bottom flask and mantle, or an aluminium block with the G10 andSiC10 vessel were unsuccessful and produced spherical nickelnanoparticles.

To account for the initial heating ramp rate, a reaction solution at100° C. was injected into a preheated G10 vessel at 290° C. in a sandbath, but the resulting reaction did not produce nanoparticles, evenafter 10 minutes. These results suggested that kinetics and heating modeof the solution are important factors, at least in this example, for theformation of nickel multipods.

Nickel precursor concentration effect: The effect of Ni(acac)₂concentration on pulsed-power multipod evolution was studied. SEM imagesof isolated materials from pulsed microwave reaction (4 pulses of 300 Wafter achieving 280° C.) with ratios of 0.75:50:10 Ni(acac)₂:OAm:OAc molratio, 1.5:50:10 Ni(acac)2:OAm: OAc mol ratio, (C) 3:50:10Ni(acac)₂:OAm:OAc mol ratio, and 6:50:10 Ni(acac)2:OAm: OAc mol ratiowere collected and analysed.

Nickel precursor type effect: The effect of Ni precursor choice onpulsed-power multipod evolution was analyzed. SEM images were collectedof isolated materials from pulsed microwave reactions (4 pulses of 300 Wafter achieving 280° C.) with ratios of 1.5:50:10 Ni(acac)₂:OAm:OAc molratio using (A) Ni(acac)₂, (B) Ni(acetate)₂ and (C) NiCl₂.

1. A method of forming metal multipod nanostructures, the methodcomprising: providing a mixture disposed in a microwave reaction vessel,the mixture comprising a metal acetylacetonate, a reducing agentcomprising a C₁-C₃₀ hydrocarbyl-amine, and a carboxylic acid of formula(A)

wherein R is a monovalent C₁-C₃₀ hydrocarbyl; and contacting the mixturewith microwaves to form the metal multipod nano structures.
 2. Themethod of claim 1, wherein the contacting of the mixture with microwavescomprises: (i) contacting the mixture with a first plurality ofmicrowaves effective to heat the mixture to a first temperature, (ii)reducing the first temperature of the mixture to a second temperature,and (iii) contacting the mixture with a second plurality of microwavesfor a time effective to heat the mixture to the first temperature. 3.The method of claim 2, further comprising repeating steps (ii) and (iii)from 1 to 8 times.
 4. The method of claim 2, wherein the firsttemperature is about 260° C. to about 300° C.
 5. The method of claim 2,wherein the second temperature is about 220° C. to about 250° C.
 6. Themethod of claim 2, wherein the time effective to heat the mixture to thefirst temperature with the second plurality of microwaves is about 1minute to about 20 minutes.
 7. The method of claim 1, wherein thecontacting of the mixture with microwaves comprises: contacting themixture with a first plurality of microwaves effective to heat themixture to a first temperature, and contacting the mixture with a secondplurality of microwaves effective to maintain the first temperature fora time effective to form the metal multipod nano structures; wherein thesecond plurality of microwaves comprises microwaves of differentwattages.
 8. The method of claim 7, wherein the first temperature isabout 260° C. to about 300° C.
 9. The method of claim 1, wherein thecontacting of the mixture with microwaves comprises: contacting themixture with a first plurality of microwaves effective to heat themixture to a first temperature, and contacting the mixture with a secondplurality of microwaves for a time effective to form the metal multipodnanostructures; wherein the second plurality of microwaves consists ofmicrowaves of the same wattage.
 10. The method of claim 9, wherein thefirst temperature is about 260° C. to about 300° C.
 11. The method ofclaim 1, wherein the metal acetylacetonate and the reducing agent arepresent in the mixture at a mole ratio of about 0.3:25 to about 3:25.12. The method of claim 1, wherein the reducing agent and the carboxylicacid of formula (A) are present in the mixture at a mole ratio of about25:2 to about 25:8.
 13. The method of claim 1, wherein the metalacetylacetonate comprises a Ni acetylacetonate.
 14. The method of claim1, wherein the metal acetylacetonate is selected from the groupconsisting of a Pt acetylacetonate, a Pd acetylacetonate, a Cuacetylacetonate, a Co acetylacetonate, a Au acetylacetonate, an Feacetylacetonate, and a Rh acetylacetonate.
 15. The method of claim 1,wherein the C₁-C₃₀ hydrocarbyl-amine is an unsaturated C₁-C₃₀hydrocarbyl-amine.
 16. The method of claim 1, wherein the C₁-C₃₀hydrocarbyl-amine is oleylamine.
 17. The method of claim 1, wherein thecarboxylic acid of formula (A) is oleic acid.
 18. A method of formingmetal multipod nanostructures, the method comprising: (i) providing amixture disposed in a microwave reaction vessel, the mixture comprising(a) nickel acetylacetonate, (b) oleylamine, and (c) oleic acid, (ii)contacting the mixture with a first plurality of microwaves effective toheat the mixture to a first temperature, wherein the first temperatureis about 260° C. to about 300° C., (iii) reducing the first temperatureof the mixture to a second temperature, wherein the second temperatureis about 220° C. to about 250° C., (iv) contacting the mixture with asecond plurality of microwaves for a time effective to heat the mixtureto the first temperature, (v) reducing the first temperature of themixture to the second temperature, and (vi) contacting the mixture withthe second plurality of microwaves for the time effective to heat themixture to the first temperature to form the metal multipod nanostructures; wherein the time effective to heat the mixture to the firsttemperature with the second plurality of microwaves is about 1 minute toabout 20 minutes.
 19. The method of claim 18, further comprisingrepeating steps (v) and (vi) from 1 to 20 times.
 20. The method of claim18, wherein the metal multipod nanostructures include arms having anaverage aspect ratio of about 3 to about 5.